The ability of therapeutic agents to inhibit glioma growth is drastically limited by the development of resistance. Currently, clinical trials using anti-angiogenesis therapies are showing encouraging results. However, clinical practice reveals that cancer patients initially responding to angiogenesis inhibitors their tumors elicit an evasive resistance. Pre-clinical and clinical data further implicate angiogenesis inhibition as a driving force in tumor progression to stages of greater malignancy, reflected in heightened invasion and tumor dispersal into surrounding tissue. The molecular and cellular events underlying the invasion-based tumor recurrence are incompletely understood, and further preclinical studies should be warranted to elucidate the mechanisms of this adaptive-evasive resistance, so as to design and test the potential of mechanism-based combination therapies aimed at impeding this insidious consequence of singular antiangiogenic therapy. Our preliminary data show that, following anti-VEGF treatment, the tumors acquired a new phenotype that was characterized by migration, infiltration and aggregations of glioma cells far from the original tumor mass. In addition, we observed a dramatic accumulation of Tie2+ monocytes (TEMs) in the tumor areas undergoing extracellular matrix remodeling. Co-cultures of glioma cells and TEMs showed increased migration and invasion capabilities in vitro. In addition we have set up a novel physiologically-relevant human biomatrix culture system to examine the invasion of glioma cells when co-cultured with TEMs in 3D conditions. We have also established a transgenic animal model to unequivocally determine the role of the stroma in tumor invasion. These findings suggest that recurrence after anti-VEGF treatment is determined by the ability of tumors to prime and recruit TEMs. To test our hypothesis and achieve the objectives of this project we propose the following Aims: Specific Aim 1: Determine the effects of anti-VEGF therapy on host cell infiltrates in intracranial xenografts. Specific Aim 2: Analyze the phenotype and functional characteristics of Tie2 expressing monocytes in tumors refractory to anti-VEGF therapies. Specific Aim3: Examine in vivo the role of TEMs in the tumor refractoriness to anti-angiogenesis treatment. This work is highly focused on establishing a functional mechanistic link between TEMs and tumor dispersal in brain tumors treated with anti-VEGF therapies. Because the recurrence of tumors treated with anti-VEGF therapies is characterized by heighten invasion, stroma cell-intrinsic or treatment-induced expression of pro-invasion factors during tumor progression causing tumor dispersal might be implicated. Identifying the tight control of invasion by stroma cells should provide new therapeutic avenues as tumor stroma is considered an emerging target for cancer therapy. PUBLIC HEALTH RELEVANCE: Anti-angiogenesis drugs are among the most promising agents to treat brain tumors; however, potent angiogenesis inhibition alters the natural history of tumors by increasing invasion and promoting tumor dispersal. We believe that interactions of glioma cells with monocyte-infiltrating tumors expressing a key cellular receptor termed Tie2 are the basis for the heightened invasiveness. Results obtained from this proposal should propel the development of new therapies combining the targeting of vascular structures in the tumor and the Tie2+ monocytes and, as a result, improve the effect of anti-angiogenesis therapies and the prognosis of malignant brain tumors.